The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 7
Figure 7-7. A matrix-encoded keypad.
style keypads are widely available and versatile, since they have all 10 digits plus two keys
(* and #) that you can designate for special functions.
Decoding Unfamiliar Keypads
You can find surplus keypads for a dollar or so each. These usually include no explanation
of their pin connections, so it’s up to you to figure out how to decode them.
To decode an unknown keypad, you need only an ohmmeter and a pencil and paper to record
your findings. To determine the key connections one by one, begin by clipping an ohmmeter
lead to one of the keypad’s connector pins. Set the ohmmeter to a low scale, such as 200
ohms.
Press a key and hold it down while touching the other ohmmeter lead to each of the remaining
keypad pins in turn. When the ohmmeter reads a few ohms or less, you’ve found the pins
that correspond to the key in question. If you find no connection, move the first ohmmeter
lead to a different pin and repeat the procedure. When you find the two pins that connect,
write down their numbers and the key they correspond to.
Follow the same procedure for the other keys. As you progress, you may detect a pattern
that makes it easier to guess which pins will correspond to each keypress. Some keypads
don’t seem to correspond to any obvious layout, however. When you know how the keypad
decodes, you can wire the connections and write your programs to match.
118 The Microcontroller Idea Book